The present invention relates in general to power supplies for electronic and computer systems employing backup power capability in the form of super-capacitors which allow the power supply to continue to function after main power has failed.
As electronic and computerized systems are increasingly used in all aspects of daily life, so has the requirement for such systems to be available twenty-four hours a day and seven days a week, and to be able to continue to function in the event of other system failures such as interruptions or failures of national grid power.
Large systems are often protected by backup power systems using battery power or diesel generators. Smaller systems such as computer servers may also protect against failure at a local level by using redundancy in the form of running twin identical servers and allowing seamless switching from one to the other (also called “failover”), or using redundant arrays of storage disks (termed “RAID”).
With storage systems, the use of solid-state disks (SSDs) is becoming more prevalent due to their advantages in terms of speed and performance. SSDs employ a form of non-volatile memory, often in the form of SSD devices known as NAND flash memory (chips), and due to the requirement to present such memory as though it were a traditional hard disk drive, a memory controller is required to perform a translation of the hard disk protocols to instructions to read and write the memory. This operation requires a memory controller to use large translation and mapping tables and other metadata to assist in this process. For reasons of speed and performance, these tables are normally read from the flash memory at initial power-on, then stored in volatile RAM memory. In order not to affect performance, the data in RAM is only periodically flushed back to flash memory and when the system is properly shut down.
An SSD is therefore exposed to a window of time wherein the user data and the translation and mapping tables and metadata are not consistent. If the power to the drive were to be suddenly removed from the drive during this time, the up-to-date translation/mapping tables and metadata in RAM may be lost and unrecoverable. The result may be that when the SSD next powers up, it may not be possible to reconcile the user data and translation/mapping tables and metadata stored in flash memory, and user data may be lost.
To prevent data loss, it is now common to employ a backup power supply for an SSD, generally located on the same circuit board as the memory controller and flash memory devices. This severely limits the size and amount of power the backup supply can provide and batteries are generally too bulky.
However, the main requirement is to supply enough power to keep the SSD running for enough time to allow it to flush all the unsaved data from RAM to the flash memory, so that the metadata can be brought into synchronization with the user data. This has allowed for the use of relatively small backup power components, for example, capacitors commonly known as super-capacitors. Super-capacitors have capacitance values up to 10,000 times that of normal electrolytic capacitors, but much less power capacity (e.g., about 10%) of a conventional backup battery. Super-capacitors do, however, have a much higher energy density than a conventional backup battery and can therefore satisfy the requirement for a short term backup supply in a small volume that can be readily fitted into an SSD package.
Power circuits have therefore been developed for SSDs which merge super-capacitors with a DC to DC converter which supplies the various regulated DC voltages (such as 3.3V) required by the SSD, its controller and the flash memory, from the incoming main power supply voltages, which are typically 5V and 12V.
These circuits generally employ sequencing of the application of power using a power management device such that the super-capacitors are powered up first, then the DC-DC converter, and finally the output regulated voltages to the components.
A major consequence of adding super-capacitors is that extra inrush current limiting must be employed as the large values of capacitance and low internal resistance means that large currents will otherwise flow over a relatively long time as the super-capacitors charge up at power on. The current limiting may need only consist of a resistor and diode
Without super-capacitors, some form of current limiting or slow start-up circuitry may still be required for the DC-DC converter, although while the input capacitance of the DC-DC converter will be less, the solution should be capable of supplying sufficient current for normal operation whilst not consuming power, which will involve a more sophisticated and expensive solution than a simple resistor.
Current limiting is known in the prior art is for connecting a power supply to a DC-DC converter and may be based on using current limiting resistors in parallel with switches (mechanical and electronic) such as in U.S. Pat. No, 5,087,871 (Losel) and U.S. Pat. No. 6,646,842 (Pan et al).
There is an ongoing need for systems and methods suitable for reducing an inrush current for a power supply with both backup super-capacitors and a DC-DC converter.